Recently, separation of a specific gas from a mixture of gases and purification (or refinement) of various gases etc. are actively studied with a polymer membrane as a gas separation membrane. For example, preparation of oxygen-rich air by using a gas separation membrane having high oxygen permeability, and usage of the oxygen-rich air in the fields of such as medical services and fuel systems etc. are studied. Therefore, the gas separation membrane is required to have high permeability to the specific gas and to have high selectivity of the gas compared to other gases. Moreover, such gas separation membrane is also required to be excellent in heat resistance, chemical resistance and strength etc., if the properties are necessary for the environment in which the membrane is used. Polyimides are often studied as a polymer for the gas separation membrane (please refer to Patent Documents 1-3).
On the other hand, it is reported that when permeation coefficient of a specific gas is improved with regard to a polymer membrane, selectivity of the gas to the other gases (separation factor of the gas) is lowered, that is, there is a trade-off relationship between the permeation coefficient and the separation factor, and as a result there is an upper limit boundary area (limiting line of upper limit for gas separation) (please refer to Non-patent Documents 1 and 2).
It is one object to obtain a polymer membrane having performance over at least one of the limiting lines of upper limit for gas separation reported by the Non-patent Documents 1 and 2. Intensive studies are carried out so as to obtain polymer membranes having such performance. Moreover, it is necessary to study various polymer membranes other than the polyimides, which is interesting academically and industrially.